Intracranial operations, such as those involving vascular malformations, aneurysms, and certain tumors, e.g., acoustic neurinomas or skull base tumors require intraoperative retraction of the normal brain in order to obtain surgical access to deep pathological intracranial lesions. In certain situations, such as subarachnoid hemorrhage from aneurysms or cerebral edema from a large brain tumor, not only is retraction needed for surgical access, but the brain itself is swollen from the pathology itself. This requires even more retraction in order to obtain the same surgical exposure that could have been obtained from a “relaxed” brain. In essence, brain retraction is a “necessary evil” in brain surgery. Brain retraction is needed to gain exposure, but undue pressure results in increased morbidity and even potential mortality. In order to obtain adequate operative exposure, a surgical instrument known as a retractor i.e. “soft brain retractor” or “self-retaining” retractor is used. The current technology that is used in the operating room is a passive metallic system. Brain retractors are currently made of steel and usually have tapered ends. They are typically attached to a “snake” which can be tightened at the attachment to a Mayfield head holder. They come in various shapes and sizes, are “malleable” in that they can be bent to various angles depending on the procedure at hand, and can be detached and repositioned at the surgeon's discretion. The neurosurgeon places the retractor on the brain, and periodically “loosens” it or places it on another part of the brain that requires retraction. There is no feedback that is given to the surgeon on whether a local region of the brain is injured or has the potential of suffering injury. As a result, a portion of the brain can be retracted for a period of time (up to several hours) with resultant retractor injury from direct local pressure on the brain tissue, exerting local cytotoxicity or regional ischemia. It is estimated that up to 5% of intracranial aneurysm surgery, and 10% of skull base surgeries result in retraction injury to the brain.
Although different types of brain retractors with various configurations of neuro monitoring have been devised, none of them have been popularized for generaluse. Innovations in brain retractors have typically focused on the brain retractor material, the degree angulation's that can be achieved, and the neurophysiological monitoring that is applied adjacent to the retractor. Some investigators have advocated titanium, for the retractor blade, carbon grade (for lucency), and placement of gelatin sponges as buffer between the brain and the retractor. Others have advocated the use of thinner diameter blades to minimize focal brain injury; thin tapered blades are currently the favored shape. Various neurophysiological probes have been placed adjacent to the retractors to measure the effects of the retractor blades on the adjacent brain. Various parameters have been measured including EEG, evoked potentials (EP), focal cerebral blood flow measurements, and strain gauge.
The following is a summary of the prior art that has been filed on brain retractors.
A comprehensive study of the subject introduced by Andrews R. M.D. & Bringas J. Titled A Brain Retraction and Recommendations for Minimizing Intraoperative Brain Injury Neurosurgery 36(6) December 1993, p 1052-1064 survey the use of surgical retractor and the problem of brain retraction injury. In the efforts of identifying the incidence of brain retraction injury the study review the existing art and a critical approach to the technique and procedures employed. As a result of the recommendations noted by the study, local real time electrophysiological sensing and monitoring with the capabilities of predicting the state of the tissue in the examination will improve the art of intracranial operations, while reducing injury and post operative morbidity.
Several methods have been developed to try to measure and report the pressure as well as EEG signals in order to alert or report the potential harmful conditions of too much pressure applied to the brain tissue by the retractor. The prior art listed below is representative of the efforts by the community of physician and inventors to reduce the percentage of injuries resulting from current practice.
McEwen, et al. in U.S. Pat. No. 5,201,325 teach an apparatus useful in surgery for holding retractors and other surgical instrumentation in a number of different positions required by a surgeon for the performance of a surgical procedure, including advanced sensing and regulation of retraction pressures and position; and incorporating a force amplification method to drive a locking mechanism in the supporting structure that utilizes a constrained, substantially incompressible, flexible solid material to yield a mechanism that is suitable for clinical use.
Larnard in U.S. Pat. No. 6,733,442 describe an invention “Accessory for surgical instrument” The device provides a surgical device for decreasing the trauma imposed on soft tissue by extended contact with a surgical device during a surgical procedure by thermally treating the tissue. To thermally treat the tissue, the surgical device can be configured to include a structure for enveloping and receiving at least a portion of the surgical device, where the structure is configured to control thermal energy transfer between the structure and the tissue.
Brockway, et al, U.S. Pat. No. 6,296,615 describe a Catheter with physiological sensor. The disclosed embodiments present improved catheters with physiological sensors. In one embodiment, the catheter includes a pressure transducer/electronics assembly connected to a pressure transmission catheter. The pressure transmission catheter includes a hollow tube made from a low compliance material. The distal end of the hollow tube is filled with a gel-like material or plug which acts as a barrier between the catheter liquid and the target fluid. The hollow tube is partially filled with a low viscosity liquid and is in fluid communication with the gel-like material and the pressure transducer. The pressure of the target fluid is transmitted to the liquid in the hollow tube through the gel-like material and/or the wall of the distal tip and is fluidically transmitted to the pressure transducer. The pressure transmission catheter is capable of being inserted into a vessel lumen or inserted into a lumen of a therapeutic or diagnostic catheter for biomedical applications. This provides the ability to directly measure the pressure effects of the treatment catheter. In another embodiment, the distal end of the pressure transmission catheter may be electrically conductive so as to detect and transmit an electric signal. Thus, in this embodiment, the catheter can be used to detect a physiological signal.
Huey, et al. in U.S. Pat. No. 6,104,941 describe a physiological sensor with a combination of a pressure control and a catheter including an elongate member, a sensor mounted on the catheter and adapted to be placed in pressure engagement with tissue for sensing signals resulting from physiological phenomena and an expandable member mounted on the elongate member for maintaining the sensor in contact with the tissue. A pressure source is connected to the expandable member for maintaining the expandable member inflated and a pressure controller coupled to the expandable member for maintaining the contact pressure between the sensor and the tissue within pre selected limits.
Fischell et al. in U.S. Pat. No. 6,061,593 disclosed an EEG d-c voltage shift as a means for detecting the onset of a neurological event with a multiple electrode, closed-loop system for the treatment of certain neurological disorders such as epilepsy, migraine headaches and Parkinson's disease. Specifically, the present invention combines a multi-electrode array with sophisticated signal processing techniques to achieve reliable detection of the onset of a neurological event (such as an epileptic seizure or migraine headache) typically originating from a focus of limited spatial extent within the brain. It is highly desirable to detect an epileptic seizure at least 5 seconds before the onset of clinical symptoms. Since there is often a d-c shift in the EEG voltage more than 5 seconds before the seizure, disclosed herein is a means for utilizing the d-c shift of the EEG for early detection of the seizure.
Mayevsky in U.S. Pat. No. 5,916,171 describe a Tissue monitor A single signal-single probe multi parameter analyzer apparatus for monitoring various parameters of the identical volume element of body tissue, which includes an input signal generator, a single signal guide which transmits input signal in, and transmits output signal out, constituting a single signal-single probe, a signal splitter which splits output signal into two or more parts, filters which separate various components of output signal, detectors which measure the different components of the output signal, a computer and an analog to digital converter; and algorithms to evaluate the data.
Chappuis in U.S. Pat. No. 5,769,781 describes Protector retractor with a handle which carries a dry cell battery which supplies current to a microprocessor which receives signals from a sensor on the end of a bill carried by a staff which projects from the handle. The signal to the microprocessor is converted to a display on the handle. The display has alarms to indicate when the retractor applies too much force to a spinal cord or when the force has been applied for too long a time.
Ayad in U.S. Pat. No. 7,153,279 describe Brain retraction sensor with an electrode device is disclosed comprising a deformable envelope, further comprising recording electrodes and a pressure recording port. The device allows for monitoring of brain retraction pressure and local cortical electrical activity including DC potential, as well as redistribution of the force applied during retraction and cushioning of the rigid edges of the brain retractor, thereby diminishing the chance of focal brain injury during surgery. Retraction pressure recorded is equal over the full area of contact. A means is disclosed for optional evacuation of air from the system to improve accuracy and fidelity of the pressure measurements. Local brain hypothermia may be induced via the bladder and attached catheter, thereby providing additional neuro-protection during brain retraction. The device also allows for measurement of intracranial pressure, DC potential, EEG and, optionally, other physiologic parameters in epileptic and severe head trauma patients for management of edema and injury.
In the ensuing paragraphs we highlight the fact that cellular etiology do provide us with electrophysiological indications that if captured early within the time domain of the detecting procedure will enable the measuring system to predict and alert the surgeon of the impending damage to the tissue in question. We further instruct in this application that the use of the apparatus proposed solve these and other problems associated with intracranial intervention, and by the consistent application of the methods and embodiments of this invention a robust predictive algorithm is enabled so as to dramatically reduce the incidence of morbidity and mortality associated with the use of brain retractor. While use of these retractors, often for several hours, is necessary to expose the surgical site, surgeons and particularly neurosurgeons have worried that the pressure exerted on the delicate neuralgic tissues can cause irreversible damage thereto. As reported by the medical practitioner, the neurosurgeon has traditionally relied on his experience and tactile sensory outputs in setting a safe level of retractor pressure. This may prevent physical crushing or mechanical damage of the tissues, but of greater concern is the possibility of severely compromised local blood flow under the retractor tip. This reduced blood flow could lead to oxygen starvation of the tissue cells called cerebral ischemia. This type of damage cannot be visually detected by the surgeon, even with the use of a microscope, hence the use of varieties of sensor platform to detect and alert the physician of the impending mechanical damage resulting in ischemia.
Further evidence of physiological parameters such as EEG variations due to ischemia have been shown to correlate between brain retractor pressure and EEG wave form characteristics which could prove useful in evaluating or predicting damage caused by retractor pressure. Voorhies et al. U.S. Pat. No. 4,784,150 summarizes the findings of research conducted by Tolonen and Sulg (1981) which found that the power in the delta band (0.0 to 4.69 Hz) of the EEG power spectrum correlated inversely with regional cerebral blood flow, such that an increase of EEG power in this band could warn of impending ischemic damage. These parameters are exploited by this invention in the process of improving the predictive qualities of the proposed algorithm. Further reported by Behrens et al., “Subdural and Depth Electrodes in the Pre surgical Evaluation of Epilepsy” Acta Neurochir (1994) 128:84-87 that While knowing the amount of pressure applied, the variables that influence the threshold sensitivity of the brain to different degrees of retraction include the depth of anesthesia, systemic parameters such as blood oxygen and carbon dioxide levels, and the specific area of the brain being retracted. As a result, electrophysiological monitoring of the brain can give a more accurate indication of when the threshold for injury is being approached by analyzing the complex signal-patterns of the electroencephalogram (EEG) and somatosensory evoked potentials (SSEP). In the current method of measuring EEG, the electrodes commonly used depend on the position and placement on the scalp. Because of this, electrodes can only be placed to the extent that they do not interfere with the sterile surgical field, and obviously cannot be placed in the area of the craniotomy, which it is precisely the part of the brain that needs to be monitored. The invention and its embodiments as featured by the use of an intraoperative integrated MOSFET Sensor Array solve this and other problem of local definition of reporting on essential physiological parameters, without the compromise noted in the prior art.
Additional evidence of the need to record, report and predict the data generated by the EEG & SSEP signals while first establishing the retraction pressure of 30 mm Hg, (within the range used in clinical neurosurgery) has usually results in a 50% decrement in electric potential amplitude within 15 minutes of beginning retraction, (Andrews R J, Muto R P Neurol Res 14:12-18 1982). This facts and others will assist the proposed invention in setting the limits necessary for safe retraction pressure while maintaining an adequate separation of the tissue in question, this task and others is addressed by the use of “Look-Up Tables”, which reside in memory bank of the proposed apparatus.
The need to supplement the pressure monitoring and reporting of the brain retractor in order to reduce retraction injury by the use of local EEG monitoring is further supported by the experimental work conducted by Williams C. published under PCT WO 95/35060. Whereby the underlying mechanism of impedance variations within nervous tissue, (wherein the presence of myelinated tracts giving a relatively low conductivity), results in conductivity change of the tissue rises as the ion-containing, extra cellular fluid which provides for more conduction paths. Typical values for white matter are 700 ohm-cm; for grey matter, 300 ohm-cm. The skull is typically 5000 ohm-cm. This variation of conductivity in different tissues are the main reason why the bioelectric potentials need to be measured locally, so as to avoid the SNR (Signal to Noise Ratio) distortion associated with global EEG indications as the primary tool for predicting the anticipated event of ischemia due to over pressure, or prolonged retraction of the apparatus. In addition to differences in local conductivity between gray and white matter, the measurements from global EEG measurements are further compromised secondary to the use of medications administered at the time of surgery such as anesthetic agents, dexamethasone (given to reduce brain swelling), mannitol (an osmotic agent used for diuresis), and lasix (osmotic agent used for diuresis). Other drugs such as intraoperative anticonvulsants (i.e. phenytoin or keppra) may cause distortions in local neurophysiology. The net result, cell swelling, is really a combination of retraction pressure, medications administered, and anesthesia. Cellular swelling affects both neurons and glial cells, of which neurophysiological changes are best appreciated on a local intraoperative EEG level rather a global scalp EEG. Therefore, these cellular changes due to metabolic assimilation of mechanical as well as chemical changes are mirrored by electrical manifestations, resulting in a state which this novel MOSFET apparatus, with its local EEG, detects. Moreover, these variations and prediction of the state of the cellular conditions and or viability relative to perfusion of blood as well as oxygenation is than reported to the surgeon via audio as well as visual messages.
In addition to external factors (i.e. retraction, anesthesia, medications), intrinsic intracranial pathology may result in intracellular and intercellular fluid accumulation, resulting in decrease in tissue conductivity, with increased impedance. These changes in the cellular structure are mapped and mirrored by the corresponding electrical characteristics of the cellular medium, hence providing the physical basis for the EEG local monitoring as a predictive tool for anticipating the condition of ischemia. The MOSFET Integrated EEG/Pressure & Temperature Sensor Array enable the physician to readily obtain impedance values of the measured tissue as well as EEG data so as to improve the predictable embodiments of the use of the proposed invention.
Additional parameter which correlate the mechanical pressure exerted by the brain retractor and EEG outputs was reported by Pronk and Simons (1982), concluding that the Hjorth time domain parameter of “mobility” where Short-time segments of duration 1 s or longer are analyzed and three parameters are computed. The first parameter is called activity A2=<y2> and is simply the variance of the signal segment. The second parameter, called mobility Mx, is computed as the square root of the ratio of the activity of the first derivative of the signal to the activity of the (original) signal: <(dy/dt)2>/A2. The third parameter, called complexity or the form factor FF, is defined as the ratio of the mobility of the first derivative of the signal to the mobility of the signal itself: C2=<(d2y/d2t)2>/A2 (Hjorth, 1970). Other techniques that can possibly be employed while applying the retractor with the apparatus noted by the invention are: Time Domain Parameters, Barlow Parameters Frequency Domain Parameter, FFT, Periodogram and the Hjorth parameters noted above. The processing of the local EEG with the analytical tools noted above, is supported by the proposed architecture as described by the accompanying figures.
The apparatus is further augmented with the necessary limits for safe retraction pressure and duration by the aid of look-up-tables 603.1, residing in the memory banks of microcontroller 603. The threshold and boundary conditions for the limits, are defined by algorithm and AI routines 603.2 forming the alerts loop 603.3.
Mechanical pressure placed on the tissue by the brain retractor will results in lower blood flow immediately beneath the retractor compared to the surrounding regions. Astrup et. al. 1981, have found that flow rates below 10 to 13 ml/100 gm/min lead to cell damage. It has been found that if the Mean Arterial Pressure (MAP) exceeds the Brain Retraction Pressure (BRP) by less than 70 mm Hg, the brain will be damaged (i.e., brain damage will occur if (MAP-BRP<70 mm Hg)). However, it has also been found that when the difference between BRP and MAP is greater than 100 mm Hg, the brain will typically recover completely (i.e., no lasting brain damage will occur when (MAP-BRP>100 mm Hg).